Their faces extend in elongated snouts, and two lower teeth grow upward into horny tusks, making beaked whales one of the more fanciful creatures to prowl the deep.

Although they’re the most diverse marine mammals next to dolphins, you’re not likely to ever see one. These whales are stealthy, deep-diving, and according to a new study, possibly declining.

The study, by researchers Jeffrey Moore and Jay Barlow with the Southwest Fisheries Science Center, was published last week in the online science journal PLOS ONE. It analyzed nearly two decades worth of data on the whales to estimate their numbers.

The researchers concluded that while beaked whale populations are hard to count, they’re probably dwindling, and Navy sonar and changes to ocean chemistry may be to blame.

“We have high certainty that those have been declining,” Moore said.

Navy officials said they’re reviewing the study, but said its findings are too vague to implicate its sonar operations as a cause of whale decline.

Beaked whales range from 13 to 42 feet long, with narrow, beak-like snouts that inspired their names They’re members of the group of toothed whales, along with dolphins and porpoises, but have only two teeth. Those evolved into protruding tusks that males use for fighting and luring mates.

“They’re really pretty strange looking,” he said.

Aside from dolphins, beaked whales are the most varied group of marine mammals, with at least 21 different species. Yet they’re so rare that some have been identified only by remains.

“There are a few species that have never been seen alive at sea,” Moore said. “They’re only known from a few carcasses that have washed offshore, or from a single skull or single specimen.”

The whales haunt the ocean depths, diving more than half a mile for prey and surfacing rarely, Moore said, which makes them hard to spot and even harder to study.

“As common as they might be, they are, as a whole, kind of a mysterious group,” he said.

To measure their populations, scientists relied on data from sweeping, months-long transects of the California current. Every few years since 1991, the fisheries center has embarked on research cruises to survey the coast from the Canadian to the Mexican border, extending 300 miles offshore, Moore said.

Each of those cruises sighted only a small number of beaked whales, but scientists used distance-based statistical models to calculate how many whales they didn’t spot, based on the number and location of the ones they did, Moore said.

Their estimates for populations of three main groups of beaked whales ranged widely — from about 8,000 to 25,000 — in 1991. But they suggested a downward trend by 2008, with estimate for that year ranging from about 4,000 to 15,000 for beaked whales as a whole.

Moore acknowledged that the range is wide, but said the statistical probability that they’re declining is better than 95 percent.

“It’s hard to have a really precise model of how many there are, but the way the trend model works, we’re able to make strong conclusions that ... wherever it falls in that range of uncertainty, it’s going down,” he said.